1. Field of the Invention
The present invention relates to a method for the control of venetian blinds, and in particular, venetian blinds in buildings which have windows and are used for industrial or commercial purposes, as well as to a control device incorporating a light sensor, such as a photodetector or a light-sensitive cell, for controlling raising and lowering of such venetian blinds as well as for varying the screening angle of the slats of such venetian blinds.
2. The Prior Art
In the case of modern office buildings, providing protection from the sun's rays is important in preventing excessive heating of the work areas within the buildings, in saving energy in situations wherein air conditioning installations are employed and in providing display-quality worksites that are protected from glare. Up to now it has been customary to control the venetian blinds of the facade wall of an office building by means of sensors installed centrally on the roof of the building. Such an approach has the disadvantage that at any time according to the prevailing conditions, some windows of a facade wall are, under certain circumstances, completely protected from sunlight while other windows of the same facade wall lie in the shadow of other buildings or other parts of the same building. The problem of the different light exposure experienced by different windows of a facade wall is made even more difficult by virtue of the fact that the situation changes with the movement of the sun during the course of a day and according to the time of year. More recently, centrally controlled sun protection installations have been provided in an attempt to improve the operation of the automatic control systems for the venetian blinds. These installations use costly shading arrangement diagrams and astronomical clocks. However, such installations do not overcome the basic drawback of prior installations, viz., that the special proportions and relationships of the lighting provided at any one window are not taken in account.
One further problem of centrally controlled venetian blinds is that, following the triggering of the lowering movement of the blinds in response to sensing a preselected background brightness, the lowering command for all of the venetian blinds remains in effect only during a predetermined time period, and following this predetermined blind lowering time period, a signal is generated which provides for opening of the blinds. It will be appreciated that if blinds of different lengths (heights) are located on a building facade wall, the lowering time of the longest blind must usually be adjusted and thus the rooms with shorter lowering times, will, as a result, remain darkened for an unnecessary length of time, before the opening control pulse is generated. The opening or screening angle of the slats which is set thereafter can thus be completely incorrect depending on the time of day and time of year and the particular position of a window.